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  • ISO is the abbreviation for a rating scale that indicates how sensitive the camera film or sensor is to light and is equivalent to the old film ASA scale
    • most films were rated at ASA 25-400 with films rated higher than 400 generally being quite grainy.
  • the lower the value, the LESS “sensitive” to light (ie. less amplification of the sensor signal) and thus you need a larger aperture, longer shutter or stronger light or a combination of these to get the same exposure if you lower the ISO value.
  • in general, photographers aim to use the lowest ISO setting possible for the conditions, subject and lens as the lower the ISO the better the image quality in terms of image noise and dynamic range
  • in low light conditions or with fast moving subjects, one may need to increase ISO to reduce camera shake or subject motion.
  • most cameras have a starting or “base” ISO of 100 or 200 although some newer cameras have a “dual base ISO” although there is still no change to the sensor's signal to noise ratio or “sensitivity”, these cameras just apply the amplification earlier in the signal chain to provide a cleaner output.
  • each time the ISO is doubled, this equates to 1 stop brighter (ie. it is twice as bright)
  • image noise increases (and dynamic range and image quality decreases) as ISO increases and the extent and type of this image noise depends upon the photosite size on the sensor and the technology used
    • most modern dSLRs give acceptable image noise up to ISO 1600
    • the latest full frame sensor cameras give reasonable image quality above ISO 6400
  • most photographers using flash or tripods will prefer to use the base ISO for the majority of their shots
  • most hand held photography is done at ISO 100-400 which allows a fast enough shutter speed to minimise camera shake in most conditions outdoor in the daytime
  • indoor hand held photography without flash often requires ISO 800 with a fast lens such as f/2.0
  • sports photography with fast shutter speeds of 1/500th second or faster often require ISO 1600 or higher.

technical aspects of ISO

  • The ISO Standard 12232 definition that ISO Sensitivity is:
    • ISO Sensitivity = 78/Luminance
  • Actual digital camera measured ISO sensitivity is usually around 0.3-0.6EV less than the stated ISO, while Olympus cameras generally have a measured ISO around 1.3EV less than stated
  • the difference in EV between two ISO values is:
    • number of stops EV = log2(ISO2/ISO1)

iso invariance

  • ISO invariance refers to some modern camera sensors which can give the same quality image whether shot at high ISO, or shot at base ISO and severely under-exposed then over-exposed in RAW development on a computer
  • this can be handy in contrasty night shots when one does not want to blow out highlights which can then be selectively brightened in post-processing more so than the darker regions

reducing high ISO noise in Lightroom

    • also shows how to optionally then edit in Photoshop to create a High Pass Filter layer to further improve the image
    • usually LR's default colour NR settings work well for most images leaving you to adjust the luminance NR settings visually:
      • set sharpening to default to start
      • zoom in on image 3:1
      • holding ALT while adjusting luminance NR level displays only the luminance noise - adjust slider until nearly all noise is gone
      • holding ALT, adjust luminance NR details slider towards right and stop when you just start to see noise
      • holding ALT, adjust luminance NR contrast slider towards right and stop when you just start to see noise
      • if changing color NR, too much smoothness will cause colors to bleed out over edges
      • set sharpening to maximum to allow visualisation of optimum settings
      • holding ALT, adjust sharpening radius to right until you see edges
      • holding ALT, adjust sharpening details to right until you see noise
      • holding ALT, adjust sharpening masking to right until you are happy that only edges are being sharpened
      • adjust the overall sharpening amount as desired

reducing high ISO noise using image stacking

photo/iso.txt · Last modified: 2018/07/08 11:21 by gary1

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